A spiral bevel gear is a bevel gear with helical teeth. The main application of this is in a motor vehicle differential, where the direction of the drive carried by the propeller shaft (drive shaft) has to be turned through 90 degrees in order to power the driven wheels. The helical design produces less vibration and noise than conventional straight-cut or spur-cut gear with straight teeth.
Spiral bevel gear belongs to a wider domain of hyperbolic gears. Hypoid gear is a common example of hyperbolic gear. The pitch surface of the hypoid gear is a hyperbolic surface. Hypoid gear becomes spiral bevel when the hypoid offset is equal to zero. On the spiral bevel gear the hyperbolic pitch surface becomes comical. Hypoid gears are commonly used in automotive rear drive axles. However today, the hypoid gears are often replaced by spiral bevel gears because of increased driving efficiency. Most of small cars do not have hypoid gears any more. But hypoid gear are still common on large trucks because they can transmit higher torque. As higher hypoid offset as higher torque the gear set can transmit. However, increasing of the hypoid offset results in reduction of driving efficiency and reduction of fuel economy. Unfortunately, it is often impossible to replace low efficiency hypoid gear with efficient spiral bevel gears because the spiral bevel gear would have to be much larger in diameter in order to transmit the same torque. In creasing the size of the drive axle gear would result in increase of the size of the gear housing and reduction of the ground clearance.
A right hand spiral bevel gear is one in which the outer half of a tooth is inclined in the clockwise direction from the axial plane through the midpoint of the tooth as viewed by an observer looking at the face of the gear.
A left hand spiral bevel gear is one in which the outer half of a tooth is inclined in the counterclockwise direction from the axial plane through the midpoint of the tooth as viewed by an observer looking at the face of the gear.
A spiral bevel gear and pinion are always of opposite hand, including the case when the gear is internal.
The designations right hand and left hand are applied similarly to spiral bevel gears, zero bevel gears, skew bevel gears, hypoid gears, and oblique tooth face gears.
In hypoid gear design, the pinion and gear are practically always of opposite hand, and the spiral angle of the pinion is usually larger than that of the gear. The hypoid pinion is then larger in diameter than an equivalent bevel pinion.1